Why do XL dogs only have a 9 or 10 year life span?

So I have a saint bernard and he’ll be 4 years old this month. It makes me sad to know that there is a good chance that he’ll only have another 5 or 6 years to live. So i’m just curious why large dogs like that only have a 9 or 10 year life span?

What is the average life span of a parson russel terrior?

I’m looking to adopt one and not sure what the average life span of this type of dog is. Thanks

How long do you think my dogs life span is?

He is a Lab mix possibly a Lab and Dachshund mix. He is sorta small but I do not know how old he is now but I know he is not a year yet

He looks like this: http://blogs.discovery.com/.a/6a00d8341bf67c53ef0133f64fd16c970b-800wi

How long do you think his life span is?